Method for producing iron carbide from granulated sponge iron

ABSTRACT

Process for producing Iron carbide wherein, in a first stage, Iron ore is reduced to sponge iron using a reducing gas containing at least 90% hydrogen, on a nitrogen-free basis, to produce a sponge iron having a carbon content of less than 1% wt.; then in a second stage the sponge iron is fluidized at a temperature of 500 to 800° C. with a methane-containing fluidizing gas in a fluidized bed reactor wherein the water content of the gas in the reactor is not more than 1.5% wt., to produce a product wherein at least 85% of the iron content is in the form of Fe 3 C.

This invention relates to a process of producing iron carbide (Fe₃C)from granular sponge iron, which comes from an iron ore reduction plantwith a carbon content of not more than 2 wt-%.

From the U.S. Pat. Nos. 5,527,379 and 5,603,748 the direct reduction ofiron oxide is known, where in several fluidized beds granular,iron-oxide-containing material is brought in direct contact with hotreduction gas at temperatures of 500 to 900° C. When the reduction gasnot only contains hydrogen, but also a considerable content of carbonmonoxide, a product rich in Fe₃C can be withdrawn from the last stage ofthe fluidized bed of the known reduction process. Practice has shown,however, that in the reduction of iron oxide to iron the steam producedgreatly impedes the simultaneous formation of iron carbide as a resultof the reaction of iron with Co and/or CH₄.

It is therefore the object underlying the invention to omit thesimultaneous production of iron carbide (Fe₃C) during the directreduction of iron oxide and the formation of sponge iron. In accordancewith the invention iron carbide is produced by means of theabove-mentioned process such that the sponge iron, which has a carboncontent of not more than 2 wt-%, is swirled in a fluidized-bed reactorat temperatures of 500 to 800° C. with a carbonaceous gas whose watercontent is not more than 1.5 vol-%, and that from the reactor a productis withdrawn, whose total iron content is bound as Fe₃C for at least 80wt-%. Preferably, at least 85 wt-% of the total iron content of theproduct withdrawn are bound as Fe₃C.

In the process in accordance with the invention, carburizing thelow-carbon sponge iron is deliberately effected separate from thereduction plant. This requires a more complex apparatus than the knownproduction of iron carbide, but the reduction plant is relievedconsiderably in the process in accordance with the invention. Now, thereduction plant is preferably operated with hydrogen-rich gas asreduction gas, which contains only little CO or is virtually free fromCO. During the carburization a H₂-containing gas is produced, and thishydrogen can advantageously be utilized upon separation in the reductionplant. It is recommended to form the reduction gas supplied to thefluidized bed of the last reduction stage from hydrogen for at least 80vol-% (calculated free from nitrogen). Then, the granular sponge iron,which is supplied to the fluidized-bed reactor for carburization,usually has a carbon content of not more than 1 wt-%.

For carburizing in the fluidized-bed reactor gases rich in hydrocarbonare used, which in the reactor may also serve as fluidizing gases. Asgas rich in hydrocarbon there might for instance be used methane ormethane-containing natural gas. To accelerate the carburization, thefluidized-bed reactor is operated at pressures in the range from 3 to 10bar. In the fluidized-bed reactor, the solids may form a stationaryfluidized-bed, or they may be held in the state of the circulatingfluidized bed. In the latter case, the reactor comprises a separator forthe separation of solids, which is connected with the upper part of thereactor, and from which separated solids are recirculated to the lowerpart of the reactor. Per hour, at least 5 times the weight of solids isrecirculated, as compared with the solids content in the reactor.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the process will now be explained with reference to thedrawing. It represents a flow diagram of the process.

From granular iron oxide, which is supplied via line 1, there is firstof all produced by means of reduction sponge iron with a carbon contentof not more than 2 wt-% and preferably not more than 1 wt-%. Thereduction may be effected in any manner known per se. An advantageousprocedure is described in the already mentioned U.S. Pat. Nos. 5,527,379and 5,603,748, where a drying and heating stage 2 is followed by a firstreduction stage 3 and a subsequent second reduction stage 4. In bothreduction stages the reduction is effected in a fluidized bed, wherehot, hydrogen-containing gas is used as reduction and fluidizing gas.The temperatures in both stages 3 and 4 lie in the range from 500 to900° C. The first stage 3 is designed as circulating fluidized bed, towhich at least in part used, H₂-containing reduction gas from the secondstage 4 is supplied through line 5. The exhaust gas of the first stageis recirculated via line 6 to a processing plant 7, in which there isalso produced fresh gas rich in hydrogen. The gas is supplied as hotreduction gas through line 8 to the second reduction stage 4, in whichthe solids preferably form a stationary fluidized bed. Preferably, thegas of line 8 comprises at least 80 vol-%, and mostly at least 90 vol-%hydrogen. A partial stream of the reduction gas of line 8 is expedientlysupplied directly to the first stage 3 through line 8 a. The degree ofmetallization of the partly reduced ore of line 3 a is about 50 to 80%.

From the second reduction stage, granular sponge iron with a carboncontent of not more than 2 wt-% and preferably not more than 1 wt-% iswithdrawn via line 10. This sponge iron is charged into a fluidized bedreactor 11, which is connected with a cyclone separator 12. Forcarburizing the sponge iron, gas rich in hydrocarbon, which for instancechiefly consists of methane, is supplied through line 13. This gas firstof all flows into a distribution chamber 14 and then as fluidizing gasthrough a tuyere bottom 15 upwards through the reactor 11. In thereactor 11 the temperatures lie in the range from 500 to 800° C. Agas-solids suspension is supplied from the upper part of the reactor 11through the passage 16 into the separator 12, and separated solids arerecirculated to the reactor 11 through line 17. The product withdrawnthrough the passage 18 from the lower part of the reactor 11 chieflyconsists of iron carbide, where at least 80 wt-% of the total ironcontent are bound as Fe₃C. This product is supplied to a cooling unitnot represented.

Solids-containing gas leaves the separator 12 through line 20 and firstgives off heat in the heat exchanger 21. Due to the carburization, thegas of line 20 has a considerable hydrogen content, so that the H₂content, calculated anhydrous, will be at least 10 vol-%. Expediently,there should be ensured a H₂ content in the gas of line 20 of 15 to 40vol-% (calculated dry). For dedusting, the gas is first supplied to afilter 23 through line 22, and is then supplied to a wet scrubbing unit25 via line 24. In the scrubbing unit 25 washing solution is sprayed inthrough line 26, and used, solids-containing washing solution iswithdrawn via line 27. Cleaned gas is sucked in via line 29 by means ofthe blower 30. It is very advantageous to at least partly separate thehydrogen content of the gas and utilize the same in the reduction plant.For this purpose, the gas is wholly or partly supplied through line 31to a separating means 32 for separating a gas fraction rich in H₂ fromthe gas mixture. If desired, a partial stream of the gas coming from theblower 30 may be guided past the separating means 32 through the bypassline 33 and the opened valve 34.

The separating means 32 may operate in a manner known per se, forinstance according to the principle of pressure-swing adsorption, or maybe designed as membrane separation. Furthermore, it is possible toeffect a gas separation by means of deep cooling. In addition to a gasfraction rich in H₂, which is discharged via line 36, a residual gas isobtained from the separating means 32, which residual gas is withdrawnvia line 37. When a purge gas is used, such as in the case of apressure-swing adsorption plant, the latter is withdrawn via line 38indicated in broken lines. The residual gas of line 37 is mixed with thegas of line 33 and fortified by gas rich in hydrocarbon, e.g. methane,from line 38 a, The gas mixture, which serves as carburizing gas, ispassed via line 37 a for heating purposes first through the heatexchanger 21 and then through the fired heater 40, before it is fed intothe reactor 11 through line 13.

When the H₂-rich gas of line 36 is already suited as reduction gas, itmay directly be admixed to the reduction gas of line 8. Otherwise, thisgas is supplied to the processing plant 7 through line 36 a indicated inbroken lines.

EXAMPLE

A procedure corresponding to the drawing involves the production of 600000 t Fe₃C per year from an iron ore with an Fe content of 67 wt-%,which comprises 96 wt-% Fe₂O₃. In addition to other gangue, the orecontains 2.4 wt-% SiO₂. The data have been calculated in part. In thefirst reduction stage 3, which is designed as circulating fluidized bed,the solids temperature is 630° C., and in the stationary fluidized bedof the second reduction stage 4 the temperature is 640° C.

The reduction gas of line 8 comprises 90 vol-% H₂ and still contains 9vol-% N2 and 1 vol-% H₂O.

The sponge iron of line 10 has a content of metallic Fe of 88.0 wt-%, anFeO content of 8.5 wt-%, and it contains 3.5 wt-% SiO₂; the carboncontent is negligibly small.

For carburizing, sponge iron is supplied to the reactor 11 in an amountof 63.6 t per hour, the pressure in the reactor is about 4 bar, thetemperature is 600° C. In the various lines, the following gasquantities flow, and their temperatures and their components CH₄, H₂ andH₂O are indicated in the table.

Line 13 20 31 33 36 37 38a Gas 128,400 135,700 49,800 85,500 14,30033,600 9,300 quantity (Nm³/h) Tempera- 730 600 85 60    40 40   25 ture(° C.) CH₄ 79.2 69.6 69.6 69.6 — 98.7   100 (vol-%) H₂ 19.8 29.5 29.529.5   100 — — (vol-%) H₂O 1 0.9 0.9 0.9 — 1.3 — (vol-%)

The separating means (32) is a pressure-swing adsorption plant, and 9300Nm³/h CH₄ are withdrawn through line 38 a. The product withdrawn fromthe reactor 11 comprises 89 wt-% Fe₃C, 8 wt-% FeO and 3 wt-% Sio₂.

What is claimed is:
 1. A process of producing iron carbide (Fe₃C) fromgranular sponge iron, comprising the steps of (a) reducing iron ore inan iron-ore-reduction plant, said reduction plant comprising at leasttwo reduction stages operating in series, the iron ore being fed into afirst reduction stage and partly reduced iron ore from a precedingreduction stage is fed into a last reduction stage of said reductionplant, said last reduction stage comprising a stationary fluidized bedof partly reduced iron ore which is treated at temperatures in the rangefrom 500 to 800° C. and reduction gas is supplied into said bed, saidreduction gas contains at least 90 percent by volume hydrogen, on anitrogen-free basis, granular sponge iron with a carbon content of notmore than 1 percent by weight is produced in and withdrawn from saidlast reduction stage, (b) said withdrawn sponge iron is treated incirculating fluidized bed comprising a fluidized bed reactor, aseparator connected with the upper portion of the reactor for separatingsolids, and a solids return line between the separator and the lowerportion of the reactor, where per hour at least five times the weight ofsolids, as compared to the solids content in the reactor, isrecirculated from the separator into the reactor, the sponge iron isfluidized in the fluidized-bed reactor at temperatures of 500 to 800° C.with a fluidizing gas comprising methane, the water content of the gasin the reactor being not more than 1.5 vol. %, from the separator a gaswith a H₂ content of 10 to 40 vol. %, calculated anhydrous, iswithdrawn, and part of said gas from the separator is recirculated tothe reactor upon addition of methane, and from the fluidized bed reactora product is withdrawn, whose total iron content is bound as Fe₃C in anamount of at least 85 wt. %.
 2. The process according to claim 1 whereinthe pressure in the fluidized-bed reactor in which the sponge iron istreated is from 3 to 10 bar.